Lithium ion secondary batteries are widely utilized in uses such as portable electronic devices and personal computers. In these uses, the battery is conventionally required to have the functions such as compactness and lightness. On the other hand, the increase of the energy density of the battery has been the important technical problem to be solved.
Several methods have been developed for increasing the energy density of the battery among which the increase of the operating voltage of the battery is effective. In the conventional lithium ion secondary battery in which lithium cobaltate and lithium manganate are used as a cathode active material, each of the operating voltages is a 4V-level (average operating voltage=3.6 to 3.8V: vs. lithium potential). This is because the oxidation-reduction reaction of the Co ion or the Mn ion (Co3+Co4+ or Mn3+Mn4+) defines the developing voltage. On the other hand, a 5V-level operating voltage is known to be realized to use, as an active material, a spinel compound prepared by replacing the Mn of the lithium manganate with Ni or Co. For example, as described in Electrochem. Solid State Lett. 1, No.5, 212 (1998), the use of a spinel compound such as LiCoMnO4 is known to generate a potential plateau in a region of 4.5V or more. In the spinel compound, the Mn has the valency of 4, and the operating voltage thereof is defined by the oxidation-reduction of Co3+Co4+ in place of Mn3+Mn4+.
It is difficult under the present circumstances to make the energy density of the spinel compound such as LiCoMnO4 significantly larger than that of LiCoO2. The higher capacities and the higher energy densities are required for responding to the expectation for secondary batteries from the various technical fields which will hereafter have the explosive demand, especially from the car industry.
In the spinel compound such as LiCoMnO4, the capacity decrease and crystalline deterioration at a higher temperature may take place, and the improvement with respect thereto is expected.
A method of replacing manganese and oxygen with other elements has been frequently employed for preparing the conventional 4V-level cathode active material. For example, JP-A-11(1999)-312522 and JP-A-2001-48547 disclose, for the purpose of improving the cycle performance and the storage stability at a higher temperature, the introduction of a metal such as boron together with the replacement of part of manganese of lithium manganate with cobalt. However, such an element replacement is based on the 4V-level active material composition. JP-A-2001-48547 discloses the replacement of part of Mn with another element for the purpose of improving the gradual decrease of a discharging capacity which is induced by crystalline distortion in the lithium manganate during the repetition of the uses. However, the publication describes that an amount of the replacement must be kept on or below the specified value for preventing the capacity reduction due to the decrease of trivalent Mn. On the other hand, JP-A-11(1999)-312522, referring to the technique of replacing the part of Mn with the lithium, explicitly describes the purpose of suppressing the decrease of the trivalent Mn to prevent the capacity reduction by replacing part of the above lithium with another bivalent or trivalent metal As described, the replacement of Mn of the conventional 4-V level cathode active material is based on the assumption that the valency of Mn in the active material is maintained lower for securing the capacity, and the valency of Mn is defined to be 3.635 or less in JP-A-11(1999)-312522. Since the operating voltage of the active material disclosed in the above publication is defined by the valency change of the manganese, a specified amount of the trivalent manganese is required to exist in the active material, and the composition ratio of the cobalt in the active material was generally 0.1 or less.